Crossed Molecular Beam Study of the Reaction Br + O3

6922

J. Phys. Chem. A 1997, 101, 6922-6930

Crossed Molecular Beam Study of the Reaction Br + O3† Jingsong Zhang,* Tzong-Tsong Miau, and Yuan T. Lee Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, and Department of Chemistry, UniVersity of California, Berkeley, California 94720 ReceiVed: March 6, 1997; In Final Form: May 29, 1997X

The reaction of ground-state Br(2P3/2) atom with ozone molecule has been studied by the crossed molecular beams technique at five different center-of-mass (CM) collision energies ranging from 5 to 26 kcal/mol. Product CM translational energy distribution and angular distribution have been derived from experimental data. The average product translational energy ranges from 30 to 60% of the total available energy. BrO product is forward and sideways scattered with respect to the Br atom in the CM frame. With the increase of collision energy, the fraction of the total energy channeled into products’ translation is increased, and the BrO product is scattered to a more forward direction. The product translational energy release depends strongly on the CM scattering angles, with the translational energy in the forward direction larger than that in the backward direction. The Br + O3 reaction is a direct reaction, and the Br atom most likely attacks a terminal oxygen atom of the ozone molecule. Detailed comparison of the experimental results for the Cl + O3 and Br + O3 reactions shows that these two reactions have very similar mechanisms. Ozone electronic structure plays a central role in determining the reaction mechanisms.

I. Introduction The reaction Br + O3 f BrO + O2 is important in stratospheric chemistry along with the reaction Cl + O3 f ClO + O2.1 They play the key roles in catalytic ozone destruction cycles. The ClO dimer and ClO/BrO mechanisms in which these two reactions are involved are responsible for most of the Antarctic stratosphere ozone loss.2-6 The reaction Br + O3 is one of the initial steps in the ClO/BrO cycle of ozone destruction. A large number of kinetic studies on ozone reactions with radicals such as Cl + O3 and Br + O3 have been carried out.7-13 Preexponential factors of these reactions were found to be very similar and insensitive to the reaction exothermicity;10,12,13 the rate coefficients for these reactions correlated with electron affinities of the radical atoms instead of the reaction exothermicity.10 It was suggested that the transition-state structures of these reactions were insensitive to the radical, and these reactions proceeded via early transition states that best resembled the reactant ozone.10,11,12 On the basis of the information from the kinetic studies, the reaction Br + O3 is expected to be very similar to the reaction Cl + O3. Vibrational excitation of the BrO product from the Br + O3 reaction was observed in a flash photolysis study of Br2-O3 system by McGrath and Norrish.14 Their flash photolysis light was filtered by a soda glass filter so that only the Br2 molecule, and not the O3 molecule in the Br2-O3 mixture, could be dissociated. BrO absorption was observed after a time delay of several µs. The V′′ ) 0 progression of BrO absorption was predominant, but BrO absorption bands with V′′ up to 4 were also visible in the experiments. Similar to the Cl + O3 reaction, the BrO product had considerable vibrational excitation. There has been almost no theoretical study on the Br + O3 reaction. However, due to the similarity of the Br + O3 and the Cl + O3 reactions, the semiempirical study of the Cl + O3 † This paper was originally submitted as part of the Y. T. Lee Festschrift issue [J. Phys. Chem. A 1997, 101 (36)]. * Present address: Department of Chemistry, University of California, Riverside, CA 92521-0403. Fax: 909-787-4713. E-mail: jszhang@ucrac1. ucr.edu. X Abstract published in AdVance ACS Abstracts, August 1, 1997.

S1089-5639(97)00860-8 CCC: $14.00

reaction by Farantos and Murrell15 may provide some insight into the reaction mechanism. An early transition state was located on the semiempirical ClO3 potential energy surface (PES).15 Classic trajectory calculations carried out on this PES at thermal collision energy showed that the ClO product was predominantly forward scattered with respect to the Cl atom in the CM system, and there was no long-lived complex formation. The calculations also showed that at thermal collision energy 49% of the total available energy went into translation of the products and there was a large amount of vibrational energy in ClO. Michael and Payne9 used the activated complex theory to calculate the preexponential factor of the Br + O3 reaction. They assumed a collinear approach and used the BEBO method to determine an intermediate configuration that was quite close to the reactants. More information was needed to accurately estimate the bending frequencies; therefore, the comparison between the calculated and the experimental preexponential factors was not conclusive. We have previously reported a crossed molecular beam study of the Cl + O3 reaction.16 A large product translational energy release and sideways and forward ClO scattering with respect to the Cl atom were observed. It was concluded that the Cl + O3 reaction proceeded as a direct reaction and the Cl atom most likely abstracted a terminal O atom of the ozone molecule. In this work, we extend the crossed molecular beam study to the Br + O3 reaction, which was carried out at five collision energies. With the detailed experimental information such as the product CM angular and translational energy distributions, we would like to compare these two systems and further probe the mechanisms of the reactions of atomic radicals and ozone molecule. II. Experimental Section The experimental setup in this study is similar to that in the Cl + O3 study.16 The universal crossed molecular beam apparatus has been described in detail previously.17,18 Supersonic bromine atom and ozone molecule beams were crossed at 90° in a scattering chamber under a vacuum of ∼10-7 Torr. The scattered products were detected by a differentially pumped mass spectrometric detector which rotated in the plane of the © 1997 American Chemical Society

The Reaction Br + O3

J. Phys. Chem. A, Vol. 101, No. 37, 1997 6923

TABLE 1: Experimental Beam Parameters beam condition

peak velocity (Vpk) (m/s)

speed ratio (V/∆V)

Br (6% Br2 in He) Br (9% Br2 in He) Br (9% Br2 in Kr) O3 (7% in He) O3 (7% in Ar)

2370 1900 1020 1280 640

5.7 5.8 7.7 13.6 12.6

TABLE 2: Experimental Conditions Br Vpk (m/s)

O3 Vpk (m/s)

collision energy Ecoll (kcal/mol)

∆Ecoll/Ecoll (%)

∆Ecoll/Eavl (%)

2370 1900 1900 1020

1280 1280 640 640

26 18.5 14.5 5

27 23 31 19

12 9 10 3

two beams. The typical electron energy of the electron impact ionizer was 180 eV, and the ion energy was 90 eV. The size of the collision zone was 3 × 3 × 3 mm3, and under normal conditions the whole collision zone was viewed by the detector. Bromine atom beam was produced by thermal dissociation of Br2 in rare-gas mixtures in a resistively heated high-density graphite nozzle source.19 The Br2/rare gas mixtures were generated by passing He, Ar, or Kr through liquid bromine (Fisher or Mallinckrodt, reagent grade, without any further purification) in a glass bubbler held at an ice/water bath (total pressure 700 Torr, Br2 vapor pressure ∼60 Torr at 0 °C). For the highest collision energy in this experiment, helium gas was passed through the Br2 bubbler held at -9 °C (Br2 vapor pressure 40 Torr, total pressure 700 Torr) in a low-temperature bath. The high-temperature graphite source had a nozzle of 0.12 mm diameter and was heated to ∼1700 °C. A very large fraction of Br2 (g97%) was thermally dissociated, as confirmed by mass spectrometric measurements. Ozone beam has been described in detail previously.16 7% ozone/rare gas mixtures with 300 Torr total pressure was expanded through a 0.12 mm diameter nozzle. The nozzle tip was heated to ∼80 °C to minimize ozone dimers. Velocity distributions of the Br and O3 beams were measured by the time-of-flight (TOF) technique. The beam parameters were obtained from fitting the experimental TOF spectra by using the program KELVIN,20,21 which convoluted over the known apparatus functions. Typical beam parameters in this experiment are given in Table 1. Collision energy was varied by seeding Br2 and O3 in different rare gases and by changing Br2 concentration in the gas mixtures. Most-probable collision energies Ecoll and the spread of collision energies are listed in Table 2. Product TOF spectra from the reactive scattering were measured by using the cross-correlation method with a time resolution of 5 µs/channel.22 The nominal flight path was 30.1 cm. BrO product was monitored. The mass spectrometer was set at m/e ) 95 with low resolution to detect 79BrO isotope species, while a small amount of 81BrO was collected as well. Total counting times ranged from 0.5 to 6 h per laboratory angle. BrO product laboratory angular distributions, except for the experiment at Ecoll ) 18.5 kcal/mol, were measured by modulating the ozone beam with a 150 Hz tuning folk chopper. Signals with ozone beam on and off were recorded and the difference gave the net reactive signal. Total counting time per angle ranged from 3 to 10 min. For Ecoll ) 18.5 kcal/mol, BrO angular distribution was obtained by integrating the TOF spectra at various laboratory angles. The main scattering chamber was lined with a liquid nitrogen cooled cold panel along the walls and an additional cryogenically cooled copper panel was placed against the differential wall

Figure 1. Energy level diagram of the Br + O3 reaction. Thermodynamic values are derived from refs 13b, 25, and 26. The solid lines stand for collision energies in the experiment.

and facing the detector. These arrangements were effective in reducing the BrO background for both TOF and angular measurements. III. Results and Analysis Laboratory angular and TOF distributions were recorded at five CM collision energies from 5 to 26 kcal/mol (Figure 1). Experimental conditions for four collision energies are listed in Table 2. Newton diagrams of these four collision energies are shown in Figures 2, 6, 10, and 13. The circles stand for the maximum CM recoil velocity of the BrO product if all the available energy channels into products’ translation. Angular and TOF distributions were recorded at m/e ) 95, corresponding to 79BrO+. Laboratory angular distribution and TOF spectra were fitted by using a forward-convolution method. The FORTRAN program is an improved version based on a previous program.23 The goal of the analysis is to find the product angular and translational energy distributions in the CM frame. It starts with a trial form for the CM product flux-energy distribution. The program transforms this trial CM flux distribution into the laboratory flux distribution; it then generates the laboratory angular distribution and TOF spectra, after convoluting over the measured beam velocity distributions and the known apparatus functions such as the spread of collision angles, the detector acceptance angles, and the length of the ionizer. The program scales the calculated spectra to the experimental data and makes the comparison. This procedure is repeated so as to optimize the CM flux distribution iteratively until a best fit for the experimental data is found. Initially we tried to fit the experimental data by using an energy-angle separable form for the CM flux distribution. In this trial form, the CM flux distribution was expressed as a product of T(θ), the CM product angular distribution, and P(ET), the CM product relative translational energy distribution. As in the Cl + O3 study, a separable form of the CM flux distribution was inadequate. Using a single set of T(θ) and P(ET) functions that could well describe the TOF spectra at large laboratory angles (Θ > 40°), the fittings for the TOF spectra at small laboratory angles (Θ < 25°) were clearly too slow compared with the experimental data. In the CM frame, a faster forward contribution in the CM flux distribution was needed to

6924 J. Phys. Chem. A, Vol. 101, No. 37, 1997

Zhang et al.

Figure 2. Upper: laboratory angular distribution of the BrO product in Br + O3 reaction at Ecoll ) 26 kcal/mol. The filled circles are the experimental data. Error bars stand for 95% confidence limits. The solid lines are the calculated fitting curves. Lower: Newton diagram for the reaction Br + O3 at Ecoll ) 26 kcal/mol. The circle stands for the maximum CM velocity of the BrO product. The Br beam is defined as Θ ) 0° in the laboratory frame, and the ozone beam is Θ ) 90°.

make a satisfactory fit to our experimental data which had a very good signal-to-noise ratio. It seemed that the CM product translational energy release coupled with the CM angles, and it was larger in the forward direction than in the backward direction. This behavior is exactly the same as in the Cl + O3 reaction. To account for the coupling effect in a simplified way, a combination of different sets of uncoupled T(θ) and P(ET) was used. CM product flux distribution was expressed as the weighted sum of the products of different sets of T(θ) and P(ET):16 n

ICM(θ,ET) )

wiTi(θ) Pi(ET) ∑ i)1

(1)

Each Pi(ET) was normalized so that ∫Pi(ET) dET ) 1. The total CM angular distribution could therefore be expressed as n

ICM(θ) )

∫0∞ICM(θ,ET) dET ) ∑wiTi(θ)

(2)

i)1

Quite satisfactory fits to the experimental data can be achieved by using a trial Icm(θ,ET) function combined from two different sets of T(θ) and P(ET) (n ) 2 in eq 1). T(θ) was chosen in a point form, and P(ET) was chosen in a RRK-type functional form.16 Experimental and calculated laboratory angular distributions at four collision energies are shown in Figures 2, 6, 10, and 13. Representative experimental and fitted laboratory TOF spectra are in Figures 3 and 7. Total CM angular distributions and relative translational energy distributions at various CM angles are in Figures 4, 8, 11, and 14. Using the optimized CM flux-energy distribution Icm(θ,ET), the CM flux distributions

Figure 3. Laboratory TOF spectra of the BrO product at Ecoll ) 26 kcal/mol. The circles are the experimental data points, and the solid lines are the fitting.

in velocity space Icm(θ,u) [Icm(θ,u) ∝ uIcm(θ,ET)] both in contour maps and in 3-dimensional surface curves are plotted in Figures 5, 9, 12, and 15 for the four collision energies. Results for the Br + O3 reaction are very similar to those from the Cl + O3 reaction. Laboratory angular distributions are quite broad, partially due to the large product translational energy release. The large intensity at negative laboratory angles (Θ ) -20° to -10°) is not due to any impurity in the Br beam, since no m/e ) 95 signal was observed as O3 was substituted by CO2, and our data fittings gave quite reasonable reproduction of this intensity for the four different collision energies and kinematics. In the CM frame, the angular distributions are also quite broad, with large intensities for the sideways and forward scattering. The CM angular distributions do not have a forward-backward symmetry. The peak of the angular distribution shifts from 90° to 60° and finally 30° with the collision energy increased, and the peak becomes more predominant. Experimental data at Ecoll ) 26 kcal/mol, which is under the most favorable kinematics, allows us to obtain a quite confident fit for the CM angular distribution down to CM angle 10°. The intensity decrease from 30° to 10° in the CM frame at Ecoll ) 26 kcal/mol is quite obvious. At lower collision energies, the CM angular distribution within 20° is less certain; however, the trend of decrease in this region is still clear. Product translational energy release is large, which is especially evident in the TOF spectra at the laboratory angles near the CM angle ΘCM for collision energy Ecoll ) 26 and 18.5 kcal/mol. A fast and a slow peak in these TOF spectra indicate that the CM recoil velocity of the BrO product is very large. All translational energy release probabilities P(ET) peak quite far away from 0 kcal/mol; they are smooth and almost symmetric. The product translational energy is higher at small CM angles than at wide angles. With the collision energy increased, the fraction of the total energy channeled into



Figure 5. Contour map and the 3-D plot for the CM flux-velocity distribution ICM(θ,u) at Ecoll ) 26 kcal/mol.

Figure 4. Upper: total CM angular distribution ICM(θ) at Ecoll ) 26 kcal/mol. The maximum of the relative angular distribution is scaled to unit. Lower: total CM product translational energy distribution P(ET) at various CM angles at Ecoll ) 26 kcal/mol. Maximum probabilities are scaled to unit. The maximum translational energy is the total available energy at the most probable collision energy Ecoll ) 26 kcal/ mol.

translation and the width of the translational energy distribution increase; furthermore, the angular dependence of the translational energy release increases as well, i.e., the difference between the fast and slow translational energy releases becomes larger. These trends of the translational energy release are shown in Figures 4, 8, 11, 14, and 16, as well as in Tables 3 and 4. We tried to detect the reaction channel Br + O3 f BrO2 + O (Figure 1). There are two types of BrO2 isomers: asymmetric BrOO and symmetric OBrO. OBrO could be observed by the mass spectrometer,24 but the threshold of OBrO + O channel is not clear due to insufficient thermodynamic data. The BrOO molecule is less stable than OBrO (Br and O2 are bonded by only ∼1 kcal/mol),7,24-26 and it may not survive in the electron bombardment ionizer. The reaction channel BrOO + O would be open above about 22 kcal/mol collision energy; above ∼23 kcal/mol collision energy, BrOO might undergo decomposition. We detected no signal at m/e ) 111 at 26 kcal/mol collision energy. As in the Cl + O3 reaction, the reaction channel Br + O3 f BrO2 + O appears to be a very minor channel. IV. Discussion The CM product angular and translational energy distributions of the Br + O3 reaction are very similar to those in the Cl + O3 reaction, indicating that both reactions proceed through similar mechanisms. The reaction Br + O3 is also a direct

Figure 6. Same as Figure 2 but at Ecoll ) 18.5 kcal/mol.

reaction, as the CM angular distribution has the predominant forward-sideways peak and does not have the typical forwardbackward symmetry of a reaction via a persistent long-lived complex.27 The strong couplings between the product translational energy and the CM angles, as well as the repulsive translational energy release, also indicates a direct reaction mechanism. Besides the similar CM angular and translational energy distributions in the Br + O3 and Cl + O3 reactions,16 the


Zhang et al.


TABLE 3: Average Translational Energy Release 〈ET〉/Eavlb Ecoll

Eavla

CM angle 10°

CM angel 40°

CM angle 120°

(∆〈ET〉)maxc

26 18.5 14.5 5

57 49.5 45.5 36

0.62 0.58 0.54 0.39

0.56 0.52 0.49 0.37

0.43 0.41 0.37 0.33

11 9 8 2

a Total available energy in kcal/mol. b Fraction of average product translational energy at various CM angles. c Maximum difference of average translational energy release at small and large CM angles in kcal/mole.

TABLE 4: Peak Translational Energy Release ETpeak/Eavla Ecoll

Eavl

CM angle 10°

CM angle 40°

CM angle 120°

(∆ETpeak)maxb

26 18.5 14.5 5

57 49.5 45.5 36

0.64 0.59 0.54 0.35

0.57 0.53 0.48 0.33

0.41 0.38 0.35 0.30

13 10 9 2

a Fraction of peak product translational energy at various CM angles. Maximum difference of peak translational energy release at small and large CM angles in kcal/mol. b

collision energy dependence of these distributions and the couplings between the translational energy distributions and the CM scattering angles are also very similar. Essentially, dynamic information for both reactions obtained from the crossed molecular beam studies shows that these two reactions are very much alike.16 This similarity supports the suggestion from the early kinetic studies that the transition-state structures of these reactions were mostly determined by the configuration of the ozone molecule.10-12 The similarity also qualitatively agrees with the theoretical calculations, which indicated that the configuration of the reaction intermediate was like that of the

ozone molecule9,15 and is consistent with the argument by Schaefer and co-workers that ozone electronic structure plays an important role in the reaction mechanism.28 O3 molecule can be characterized as a diradical with the two unpaired π electrons in the terminal O atoms.29 Its center O atom has a closed outer shell of 8 electrons, while the terminal O atom has only 7 outer electrons with a half-filled 2pπ orbital perpendicular to the molecular plane. As argued previously,16 it is unlikely for the Br atom to abstract the central O atom, because of the high repulsion of the lone-pair electrons on the central O atom. This is confirmed by the predominant forward-sideways instead of backward scattering of the BrO product. It is also unlikely for the Br atom to insert into the O-O bond. Similar transition-state configurations of these two reactions imply that the Cl or Br atom probably does not insert to the O-O bond to make the transition-state structure quite different from that of the ozone molecule. The Br atom should abstract the terminal O atom of the ozone molecule. When the Br atom attacks the π orbital on the O3 molecule perpendicularly from above the O3 molecular plane, the interaction has a net overlap between the frontier molecular orbitals and is symmetry-allowed.30 This collision pathway has a large impact parameter and the BrO product should be scattered in the forward direction. With the increasing collision energy, the forward scattering would become stronger. However, this channel is not able to produce the significant amount of large-angle scattering, especially at high collision energies. The Br atom could attack a terminal O atom in the ozone molecule plane, leading to the sideways and wide-angle scattering via a bent transition state. In this coplanar approach, the Br atom has a large range of attacking angles; for a direct reaction, they correspond to a wide range of CM angles into which the product BrO could be scattered. Similar to the Cl + O3 reaction, two reaction mechanisms are possible in the Br + O3 reaction: an in-plane and an outof-plane approach. The rate of increase of the product translational energy with the collision energy varies with CM angles (Figures16 and 17). Specifically, the product translational energy increases faster for small CM angles than for wide CM angles; the internal energy remains essentially constant for very small scattering angels (Figure 17). These two different types of collision energy dependences may suggest two possible reaction mechanisms. At Ecoll ) 5 kcal/mol the forward scattering channel with large translational energy release seems to be nearly not open, and the forward scattering channel appears to have a higher effective reaction barrier than the wide-angle scattering channel. In a large impact parameter collision such as the out-of-plane approach, a fair amount of translational energy is tied up to rotation of the reaction intermediate and is not effective in overcoming the reaction barrier of the entrance channel, especially when the translational energy is low. Only at high collision energies, the forward scattering with large impact parameter in the out-of-plane approach starts to become important. In summary, the Br atom predominantly attacks the terminal O atom of the ozone molecule. At low collision energy (Ecoll ) 5 kcal/mol), the BrO product is mainly sideways scattered and the product translational energy is ∼30-40% of the total available energy. The coplanar collision channel seems to contribute dominantly at low collision energy. At high collision energies (Ecoll ) 14.5-26 kcal/mol), the BrO product is forward and sideways scattered and the product translational energy is about 40-65% of the total energy. The in-plane pathway gives a significant amount of sideways and forward scattering, while






an out-of-plane collision pathway may contribute to the forward scattering as well. Note that the forward scattering at high collision energies is more predominant than that in the Cl + O3 reaction, possibly due to the heavier mass of the Br atom. The similarity of the experimental results for both Br + O3 and


Cl + O316 is consistent with an early transition state that resembles the reactant ozone molecule, as suggested by the semiempirical studies15 and previous kinetic studies.10,11,12 The


Zhang et al.


Figure 14. Same as Figure 4 but at Ecoll ) 5 kcal/mol.

Figure 15. Contour plot of the CM flux distribution ICM(θ,u) at Ecoll ) 5 kcal/mol for the region where the TOF spectra are measured.

Figure 13. Same as Figure 2 but at Ecoll ) 5 kcal/mol.

internal energy of the products, though not resolved in this experiment, should be largely the vibrational and rotational energy of BrO product for a reaction via early transition state. The predominant forward and sideways scattering and the falloff in the very small CM angle region, even at the highest collision energy, suggest a quite strong repulsion on the exit channel. An impulsive energy release model31 predicts 33% of the reaction exoergicity going into the products’ translation at the reaction threshold. The extrapolation of the measured product translation energy release to the threshold gives a value of

∼31%, in good agreement with the impulsive model. Of course, a high-level quantum mechanical calculation on the Br + O3 reaction is very valuable and can provide more quantitative comparison with the results of this crossed molecular beam study. Product channels BrO(2Π) + O2(3Σg-) and BrO(2Π) + O2(1∆g) are energetically possible at all collision energies. When Ecoll is above 6.4 kcal/mol, the third product channel BrO(2Π) + O2(1Σg+) is also open (Figure 1). For the Cl + O3 reaction, early experiments have shown none or very little formation of the electronically excited O2(1∆g) and O2(1Σg+) products.32,33 The translational energy release probabilities in the Br + O3 reaction are quite smooth and extend near the maximum available energy, in consistency with a dominant ground-state O2(3Σg-) channel as well.


J. Phys. Chem. A, Vol. 101, No. 37, 1997 6929 ∼2000 K temperature. If Br(2P1/2) is highly reactive, there should be ∼10.5 kcal/mol more energy release, but no extra energy release was found in the translational energy distributions. The ground-state Br(2P3/2) atom should contribute predominantly in our crossed molecular beam experiment. V. Conclusions

Figure 16. Upper: fraction of average translational energy in the total energy at different CM angles versus collision energies. Lower: fraction of peak translational energy release at different CM angles versus collision energies.

We have studied the Br + O3 reaction by using the crossed molecular beam method at five collision energies from 5 to 26 kcal/mol. CM product angular distribution and translational energy distribution have been derived from experimental data. The product translational energy release is large, with the average ranging from 30 to 60% of the total energy. The BrO product is sideways and forward scattered in the CM frame. With the increase of the collision energy, the fraction of the total energy channelled into translation is increased, and the BrO product is scattered into a more forward direction with respect to the Br atom. There is a strong coupling between the translational energy release and the CM angles, with the translational energy release in the forward direction larger than that in the backward direction. It is concluded that the Br + O3 reaction has a direct reaction mechanism. The Br atom would most likely attack the terminal oxygen atom. The exit channel on the BrO3 potential energy surface should have a strong repulsion. A high-level quantum mechanical calculation on the Br + O3 reaction is valuable to compare with the results of the crossed molecular beam study. The detailed comparison of the results for the Cl + O3 and the Br + O3 reactions manifests that these two reactions have the similar mechanism. In the ozone reactions with atomic radicals, ozone electronic structure plays the central role to determine the reaction mechanisms. It is expected that ozone reactions with other atomic radicals such as F + O3 and I + O3 should proceed with the similar reaction mechanisms. Acknowledgment. We thank A. G. Suits and H. F. Davis for helping set up the ozone beam source. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC0376SF00098. References and Notes

Figure 17. Upper: average translational energy at different CM angles versus collision energies. Lower: average internal energy at different CM angles versus collision energies.

Two bromine spin-orbit states Br(2P3/2) and Br(2P1/2) are separated by 10.5 kcal/mol. Assuming a thermal equilibrium, only ∼3% of the Br atoms are produced in the spin-orbit excited-state Br(2P1/2) in our thermal dissociation source at

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Crossed Molecular Beam Study of the Reaction Br + O3

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